A grain-heated, dusty planetary nebula in M22

Detailed modeling of a halo planetary nebula located in the metal-poor globular cluster M22 has been carried out. Models of the far-infrared continuum emission observed by the IRAS satellite indicate that the total dust mass is about 0.001 solar mass, a large amount of dust when compared to the upper limit of 0.004 solar mass to the total H and He mass. Photoelectric emission from dust grains heats the nebular gas to temperatures about 10,000 K and accounts for the observed O III and Ne III forbidden emission lines. The present grain-heated nebular models predict a large, negative temperature gradient across the nebula. Advanced nuclear processing in the He-burning states of stellar evolution is implied by the presence of dust grains, most probably composed of C, and the observed O and Ne overabundance.